1. Field of the Invention
This invention relates to an apparatus for measuring the position of a one-dimensionally or two-dimensionally moved stage by the use of a light wave interferometer.
2. Related Background Art
It is a well-known technique to measure the position of a movable stage by the use of a light wave interferometer (generally a laser interferometer). Of light wave interferometers, an intensity modulation type interferometer causes a frequency-stabilized coherent laser beam (parallel light beam) to enter a movable mirror mounted on a movable member, perpendicularly to the movable mirror and also causes a laser beam (parallel light beam) to enter a fixed mirror mounted on a portion fixedly connected to the base portion of a stage, perpendicularly to the fixed mirror, and causes the beams reflected by the movable mirror and the fixed mirror to interfere with each other and photoelectrically detects any variation in the interference fringes thereof. Accordingly, when the stage is moved, the light and shade of the fringes vary repetitively in accordance with the wavelength of the laser beam and the amount of movement, and a photoelectric signal (of a sine wave shape) obtained at this time is converted into a digital pulse, which is measured by a counter to thereby find the position of the stage. A frequency modulation type interferometer is also known, and this endows laser beams travelling toward a movable mirror and a fixed mirror with a predetermined frequency difference, causes reflected beams from the movable mirror and the fixed mirror to interfere with each other and measures the amount of movement of a stage from the transition of the phase of a beat signal (difference frequency) obtained when the interference is photoelectrically detected.
An example of an exposing apparatus provided with a stage measured by a light wave interferometer will now be described with reference to FIG. 14 of the accompanying drawings. FIG. 14 shows an exposing apparatus of the step and repeat type or the step and scan type, and a reticle R having the original picture of a circuit pattern depicted thereon is uniformly illuminated by exposure light from an illuminating system ILS for exposure while it is held on a reticle stage RST. The pattern of the reticle R is imaged and projected onto a wafer W having a photosensitive agent applied thereto, through a projection optical system PL. The wafer W is placed on a wafer stage WST two-dimensionally movable in a plane perpendicular to the optical axis AX of the projection optical system PL. The wafer stage WST is moved on a base in the left to right direction in FIG. 14 and a direction perpendicular to the plane of the drawing sheet of FIG. 14. Although not shown in FIG. 14, the reticle stage RST and the projection optical system PL are mounted on a column which is integral with the base portion of the wafer stage WST.
As shown in FIG. 14, one (or more) beam from a laser interferometer (including a laser source) IFM is transmitted through a beam splitter BS and is projected onto a movable mirror MS fixed to the wafer stage WST, and another beam from the IFM is reflected by the beam splitter BS and is projected onto a fixed mirror (reference mirror) MR fixed to the lower portion of the lens barrel of the projection optical system PL. The movable mirror MS is a mirror to be measured when the wafer stage WST is moved in the left to right direction in FIG. 14, and the reflecting surface of the movable mirror MS is formed elongately in a direction perpendicular to the plane of the drawing sheet of FIG. 14.
Also, the interferometer IFM combines the reflected-beam from the fixed mirror MR and the reflected beam from the movable mirror MS by the beam splitter BS and causes them to interfere with each other, and includes a photoelectric detector for receiving the interference beam and a pulse converter for outputting an up-down pulse on the basis of the photoelectric signal thereof. The up-down pulse from this pulse converter is reversibly counted by an up-down counter in a stage control system STD, and the position of the wafer stage WST is measured.
Further, the stage control system STD suitably controls the output signal to a motor MT for driving the wafer stage WST, in conformity with the measured position by the up-down counter. When it receives a positioning target value (coordinates value) output from a main control system MCS, the stage control system STD moves the wafer stage WST so that the current position of the wafer stage WST detected by the interferometer IFM and counted by the up-down counter may coincide with said target value within a predetermined tolerance.
Also, the exposing apparatus of this kind is provided with a TTR (through-the-reticle) type alignment system AA1 for effecting the alignment of the reticle R relative to the optical axis AX of the projection optical system PL and effecting the alignment of the wafer W and the reticle R through the projection optical system PL, or a TTL (through-the-lens) type alignment system AA2 for aligning the wafer W through the projection optical system PL. Various types of alignment information by these alignment systems AA1 and AA2 is sent to the main control system MCS and is used to calculate a target value for the accurate positioning of the wafer stage WST.
The wafer stage WST shown in FIG. 14 generally has a movement stroke of the order of 30 cm-50 cm. However, in an exposing apparatus for a liquid crystal panel, the size of a plate as a member to be exposed and therefore, in some cases, the movement stroke amounts to the order of 80 cm. Therefore, the beam optical path from the interferometer IFM and the beam splitter BS fixed on the base side to the movable mirror MS need be long correspondingly thereto. In accordance therewith, the distance of the beam optical path to the fixed mirror MR is also determined.
Now, in the measurement using the interferometer IFM, the wavelength of the laser beam is the reference of the measurement. The actual length of a wavelength of a beam varies depending on the refractive index of a medium through which the beam propagates. Accordingly, when the atmospheric pressure varies, the refractive index of the atmosphere also varies in conformity therewith and therefore, in the laser interferometer of this kind, provision is made of an automatic wavelength compensating mechanism for detecting any variation in the atmospheric pressure by a sensor and it is practiced to correct a constant for converting a wavelength of the beam into an actual dimensional value in the order of ppm.
However, the atmosphere in the beam optical path does not cause a variation in refractive index uniformly everywhere, but when a variation in refractive index occurs locally in the beam optical path, it appears as the fluctuation of the measured value by the interferometer. For example, if a gas having a temperature difference with respect to the ambient temperature slowly crosses the beam optical path, the measured value (the value of the counter) will vary unstably within a certain range in spite of the wafer stage WST being properly stationary. When as an example, the minimum resolving power of the interferometer is 0.01 .mu.m, the amount of fluctuation by the gas having the temperature difference which slowly crosses the beam optical path may amount to the order of .+-.0.1 .mu.m in the worst case. This is 0.2 .mu.m as the fluctuation width and therefore is a value which will not stand the practical use as an apparatus for exposing a pattern of line width of the order of 0.5 .mu.m.
The influence of this fluctuation will pose problems in two cases. One of them is that a random error included in the measured value of the position during various kinds of alignment using a laser interferometer becomes great and the reproducibility of alignment is aggravated. The other problem is that when the wafer stage WST is made stationary by servo control so that the measured value by the interferometer may become a constant value, the wafer stage WST follows by a minute amount and is not accurately stationary. This appears as a thickening of the line width and as a bad resolution when the pattern of the reticle R is exposed on the wafer W. Although the resolving power may be increased by the projection optical system and the illuminating system, the best use, however, will not be made of it.
Also, a component of a relatively short period (10 Hz or more) and a component of a long period (less than 10 Hz) are mixedly present in the localized variation in the refractive index of the gas in the beam optical path, and of these components, the fluctuation of a low frequency component of 1 Hz to several Hz will particularly pose a problem. This is because a high fluctuation component of several Hz or more can be substantially neglected by reading the value of the counter of the interferometer by a computer or the like a plurality of times at very short sampling intervals (e.g. of the order of 1 msec.), and averaging those values. So, it has heretofore been proposed to reduce the fluctuation of the low frequency component by the technique of positively supplying gas (air) to the beam optical path or covering the beam optical path with a cover member.
As techniques heretofore proposed, U.S. Pat. No. 5,141,318 discloses providing an air duct for blowing out gas (clean air) perpendicularly to the beam optical path of a laser interferometer, and U.S. Pat. No. 4,814,625 discloses providing such air duct in parallelism to the beam optical path. However, where gas in blown out in parallelism to the beam optical path, the blast nozzle of the air duct need be disposed rearwardly of or near the laser interferometer and be turned to the movable mirror of a stage. In this case, if the stage is at a position farthest from the laser interferometer, the gas near the movable mirror will hardly become parallel to the beam optical path and become a turbulent flow of small flow velocity, and there may occur a convection with a laser source or the like for the laser interferometer as a heat generating source. This convection itself will not so much affect the measurement by the laser interferometer. This is because an exposing apparatus of this kind is originally preserved at a constant temperature (e.g. 23.+-.0.1.degree. C.) within an environmental chamber and therefore the influence of the convection is reduced by the temperature control of the chamber.
On the other hand, where gas is flowed from obliquely above perpendicularly to the beam optical path, the blast nozzle for the gas must be disposed along the beam optical path. Again in this case, the problem of convection may likewise arise, but the influence thereof is little because the apparatus itself is preserved within the chamber as described above. However, in a position wherein the stage becomes closest to the laser interferometer, the gas from the blast nozzle is directly blown upon a wafer or the like on the stage.
Where as described above, the gas is blown from a direction parallel to or perpendicular to the beam optical path, a blast nozzle is simply provided in a free space and therefore, the flow of the gas cannot be accurately controlled and the creation of dust by the creation of a turbulent flow or the like has also posed a problem. Particularly in the exposing apparatus of this kind, the preservation space therefor is controlled to class 10 (less than ten pieces of dust in 1 m.sup.3). Accordingly, the air in the chamber containing the exposing apparatus therein is also cleaned so as to ensure class 10 or class 100.
However, the beam optical path of the laser interferometer generally lies in the lower portion of the apparatus and therefore, if the blast nozzle is simply turned to the optical path, there may arise the problem that dust (oil mist, metal powder or the like of micron size) created in the movable portion or the frictional portion of the stage or the like by a convection (a turbulent flow) is blown up to above the stage and will soon adhere onto a wafer. Also, to give a blast to the whole optical path, an air conditioning mechanism of large capacity has been necessary and large-scale facilities including a duct mechanism had to be adopted.
On the other hand, it is also a technique effective against fluctuation to cover the beam optical path with a retractile pipe or the like, but if the beam optical path is simply covered with such a pipe, the air in the pipe may stagnate or a relatively great variation in the density of the air in the pipe, i.e., a variation in refractive index, may be caused by the expansion and contraction of the pipe resulting from the movement of the stage. There is also the inconvenience that the retractile mechanism makes the apparatus large in scale.